There are many times duringDrosophiladevelopment or in the adult when cell death contributes to a phenotype of interest (e.g.,refs. 1-5, and many others). The cell death may be primary to gene function or a secondary consequence of failure of proper differentiation or cell maintenance (e.g.,ref. 6). To detect whether cells are dying and how, there are a number of assays that can be used to determine cell viability. In general, there are two fundamental ways in which cells die: necrosis or apoptosis (7 ,8 ). Necrosis is pathological death characterized morphologically by swelling of the organelles, including mitochondria, with ultimate rupture of the nuclear and plasma membranes. In contrast, apoptosis-also called programmed cell death-is characterized by condensation of the chromatin, nucleus, and cytosol, with cell fragmentation into distinct condensed bodies (apoptotic bodies), which ultimately become engulfed and removed by surrounding cells. The mitochondria in cells undergoing apoptosis remain morphologically healthy, and the DNA is degraded into nucleosomal fragments (7 ,8 ).
